"Flexible" or "floppy" disks are frequently used as a magnetic storage medium because of their portability and low cost. These disks are called "floppy" because they tend to sag away from their center when not otherwise supported. The space between a magnetic disk and the transducer is critical for proper non-contact data recording and pick-up. It has been common practice in the art to attempt to flatten and stabilize the floppy disk during the read/write operation by rotating the disk at high speeds in close juxtaposition to a fixed, flat plate sometimes called a "Bernoulli" plate. In this way, an air bearing is formed between the plate and disk such that the gap between the media surface and the plate is held constant. The thin layer of air between the disk and the plate tends to rotate with the disk and to be thrown outwardly by centrifugal force. This creates a vacuum between the plate and the disk which tends to pull the disk close to the plate and cause it to behave in a substantially rigid manner. This magnetic disk behaving in a substantial rigid manner is then rotated in juxtaposition to magnetic read/write heads disposed very closely to the disk so that the heads "fly" on an air bearing in very close proximity to the disk. This has the advantage of allowing high data density, but without the expense associated with rigid magnetic disks.
A problem associated with these flexible disks, especially miniature flexible disks, is that they are susceptible to vibration modes in the disk media that translate to positioning errors when the disk is operating. These vibration modes are well understood in the rigid disk industry, as described in "The Complete Handbook of Magnetic Recording," 3.sup.rd Edition, by Finn Jorgensen,.COPYRGT.1988. Disk drives generally employ disks that are clamped at their centers, and free on the outside. In this configuration, a flexible disk may best be treated as a circular membrane, while a rigid disk is most similar to a rigid circular plate. Resonance frequencies are present with these spinning disks as a read/write head travels over the surface of the disk, and from vibrations introduced to the disk media from the drive hub. These resonance frequencies may cause fluttering in the disk. Note that the existence of a resonance frequency does not mean that the disk will start vibrating at that, or any other frequency, but it may, if the disk is excited. Radial vibration flexures may be present in a circular membrane, that is clamped at its center and free at the edges. These radial flexures generally behave according to a f x n vibration flexure pattern, where f represents the frequency and n is an integer (1, 2, 3, 4, . . . ). Circumferential vibration flexures may also be present in the same type of disk. These radial and circumferential vibration modes may be present individually as pure radial or pure circumferential waves, or together simultaneously, as a hybrid type of wave having both a radial and a circumferential component.
Under normal circumstances, the disk is rigidly adhered to the center hub via a curricular attachment method. This reflects any vibrations coherently back to the outer edge where the circular outer edge also reflects them coherently, thereby setting up a pattern of standing waves in the media. This circular attachment mechanism leads to disk instability because the vibration modes establish a standing wave phenomena in the circular disk media.
Another problem that results in a lack of disk stability or overall media waviness is the fact that the flexible media is normally anisotropic and develops strain in the disk media when read/write heads are used. The anisotropic media typically shows a "potato-chip" like cylindrical shaped curl or two waves per revolution (2f vertical excursion from the spinning reference frame) in an unsupported free state due to the leftover residual strain. This 2f component has an additional 1f due to any overall wedge or tilt aspect imparted by the hub attachment. This varying strain imposes an additional dynamic burden on the performance of the read write head as the media spins, particularly when the penetration, pitch and roll offsets due to manufacturing tolerances are included. For example, in small diameter disks, i.e., less than 50 mm, these offsets result in increased vertical motions, especially in the stiffer ID region when allowable offsets from nominal target values are present.
Thus, there is a need for an apparatus and system for improving disk stability by minimizing the standing wave phenomena in the disk media resulting from vibration modes in the spinning disk, and for improving disk compliance.